1. Field of the Invention
The invention concerns a process for the manufacture of flexible polyurethane foams in which there is used as organic polyisocyanate a urethane-modified mixture of di-phenylmethane diisocyanates and polyphenyl polymethylene polyisocyanates with a content of 55 to 85 percent by weight of diphenylmethane diisocyanate, an NCO content of 15 to 30 percent by weight, and a viscosity of 100 to 2000 centipoises at 20.degree. C.
2. Description of the Prior Art
The manufacture of flexible polyurethane foams is known. Toluene diisocyanates or the commercially available mixtures of 2,4- and 2,6-toluene diisocyanates are usually used as polyisocyanates. A disadvantage of this is that toluene diisocyanates, due to their high vapor pressure, are relatively strongly toxic and therefore, special precautionary measures must be taken and observed during their processing.
In order to reduce the toxicity hazard and increase the reactivity, it has been suggested to replace the toluene diisocyanates by mixtures of toluene diisocyanates and a mixture of homologous polyaryl polyalkylene polyisocyanates for the manufacture of polyurethane plastics including foams.
Although mixtures of diphenylmethane diisocyanates and polyphenylene polymethylene polyisocyanates are less hazardous due to the markedly lower vapor pressure, the use of these polyisocyanate mixtures as the sole isocyanate for the manufacture of flexible polyurethane foams has not caught on in industry. The primary reason for this is the insufficient mechanical-property level, particularly, the very low breaking elongation of such polyurethane foams. This is all the more surprising, since mixtures of diphenylmethane diisocyanates and polyphenylene polymethylene polyisocyanates as polyisocyanate components for the manufacture of other polyurethane-foam types, such as rigid foams, rigid integral-skin foams, and flexible integral-skin foams, have found wide-spread application.
According to data in British Pat. No. 874,430, flexible polyurethane foams are manufactured by reaction of polyether polyols having at least two hydroxyl groups and a polyisocyanate mixture consisting of diarylmethane diisocyanates containing 5 to 50 percent by weight of a polyisocyanate having a functionality greater than 2 in the presence of water. Drawbacks of the described process are the poor processability of the foamable polyurethane mixtures, which tend to collapse during foaming and the insufficient mechanical properties of the foams produced.